Conventionally, there have been proposed various methods for determining occurrence of knocking (a knock). For example, a magnitude of a vibration generated in an internal combustion engine is detected, and then is compared with a threshold value in order to determine occurrence of knocking. In the internal combustion engine, however, a vibration may be generated due to seating of an intake valve or an exhaust valve, in addition to the vibration generated due to the knocking. Moreover, a vibration may also be generated due to an operation of an injector (particularly, a direct injection-type injector that directly injects fuel into a cylinder) or a high-pressure pump that supplies fuel to the injector. In a case where these vibrations are detected as noise, it is impossible to distinguish a vibration generated due to knocking from a vibration generated due to noise from the magnitude of the vibration in some instances. In order to solve this problem, there has been proposed a technique of determining occurrence of knocking in consideration of a waveform shape of a vibration.
Japanese Patent Laying-Open No. 2004-353531 discloses a knock control apparatus for an internal combustion engine, and this apparatus determines occurrence of knocking through use of a waveform shape. The knock control apparatus disclosed in Japanese Patent Laying-Open No. 2004-353531 includes a signal detecting part that detects a vibration waveform signal generated in the internal combustion engine, a frequency separating part that separates the vibration waveform signal detected by the signal detecting part into a plurality of frequency components, a waveform shape setting part that sets a knock waveform shape based on a value obtained by integrating magnitudes of the plurality of frequency components separated by the frequency separating part for every predetermined interval corresponding to a predetermined crank angle, a knock determining part that determines occurrence of a knock at the internal combustion engine based on the knock waveform shape set by the waveform shape setting part, and a knock controlling part that controls an operation state of the internal combustion engine in accordance with a result of the determination by the knock determining part.
The knock control apparatus described in this patent publication separates the vibration waveform signal generated in the internal combustion engine into the plurality of frequency components. The knock control apparatus sets the knock waveform shape based on the value obtained by integrating the magnitudes of the plurality of separated frequency components for every predetermined interval corresponding to the predetermined crank angle. Thus, the knock waveform shape can be represented as a sum in respective vibration modes. The knock control apparatus can accurately determine occurrence of a knock at the internal combustion engine through use of this knock waveform shape.
Typically, a magnitude of a vibration generated due to noise attenuates rapidly as its characteristic. On the other hand, a magnitude of a vibration generated due to knocking attenuates gently as its characteristic. With the characteristics described above, normally, it is possible to distinguish the vibration generated due to the noise from the vibration generated due to the knocking from a waveform. However, in a case of a vibration which is generated due to noise and is small in magnitude, a shape of the vibration is similar to that of a vibration which is generated due to knocking in some instances. Consequently, even in a case where a vibration waveform signal is separated into a plurality of frequency components and a knock waveform shape is set by a value obtained by integrating magnitudes of the plurality of separated frequency components for every predetermined interval corresponding to a predetermined crank angle as in the knock control apparatus described in Japanese Patent Laying-Open No. 2004-353531, it is difficult to distinguish the vibration generated due to noise from the vibration generated due to the knocking in some instances. In this case, there is a possibility that the occurrence of the knocking is determined erroneously.